1. Field of the Invention
This invention lies in the field of vertically moored platforms (VMP) or other floating structures, for offshore, deepwater oil production which are connected to anchors in the sea floor by large diameter pipes commonly called riser pipes. More particularly, it concerns improvements in the manner by which the riser pipes are attached at their upper ends to the floating platform, and at their lower ends to anchor means at the mudline, such as conductor pipe set in holes driven into the sea floor. The riser pipes are maintained in tension at all times. When the platform is directly over the conductor pipes, there is no deflection in the riser pipes, and therefore no lateral stress in the riser pipes. However, as the pressure of wind, tide and current causes the platform to move laterally, there must be a bending of the riser pipes.
2. Description of the Prior Art
The vertically moored platform (VMP) is anchored by vertical pipes called riser pipes, kept under high tension. As the platform and jacket move horizontally, under the influence of wind, wave and current, the riser pipes are deformed. The high tension has a tendency to concentrate the bending deformation in the riser pipes at each end of the risers, where they extend vertically into the ground at the bottom end, and into the platform at the upper end.
These large deformations are detrimental to the risers. To distribute these deformations along the riser pipes, to decrease the maximum stresses, terminators have been designed. The terminators are sections of pipe constructed of varying diameter and wall thickness, the diameter and wall thickness both decrease from a mid-section towards each end, so that the flexibility of the end portions is greater than at the mid portion of the terminator. This variable flexibility introduced into the riser pipe system by the terminator distributes the curvature and helps appreciably to reduce the maximum stresses in the riser pipes.
Horizontal bearings have been introduced and positioned at the mid-section of the terminator, so that the terminator itself can rotate in a vertical plane throughout its axis, and, therefore, distribute part of the bending above and below the horizontal bearing, which supports the riser.